Lecture 8: Planar X-ray Flashcards
What is the typical energy range of x-rays?
100 eV to 100 keV
What is the difference between hard x-rays and soft x-rays?
Hard: greater than 10 keV energy; penetrate liquids and some solids.
Soft: less than 10 keV energy; hardly penetrate matter.
Why are hard x-rays used in diagnostic imaging?
They can go through the patient to be detected on the other side.
How are planar x-rays used to generate images?
Different tissues have different levels of absorption which changed the amounts of radiation that passes through the body to be imaged.
What part of the body absorbs the most x-rays? Why?
Bone as it is very dense.
Define the linear attenuation coefficient
A constant that characterises how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy/matter.
What is the differential form of the intensity equation?
dI = change in beam intensity
I = original beam intensity
dx = incremental distance
n = number of atoms per unit volume
σ = constant related to the probability of scattering
What is the equation for beam intensity when the linear attenuation coefficient is constant?
I(x) = intensity
I_0 = initial beam intensity
n = number of atoms per unit volume
σ = constant related to probability of scattering
x = position
µ = linear attenuation coefficient
What is the general equation for the linear attenuation coefficient?
I(x) = intensity
I_0 = initial beam intensity
dx = incremental distance
µ = linear attenuation coefficient
What two things does the linear attenuation coefficient vary as a function of?
- Tissue density
- X-ray intensity
What are the three ways in which x-rays interact with matter?
- Photoelectric effect
- Compton scattering
- Pair production
Define the photoelectric effect
When a photon interacts with an electron the electron is emitted from the atom as a photoelectron and the photon is absorbed.
What is the relation that described the contribution of the photoelectric effect to the linear attenuation coefficient?
µ = linear attenuation coefficient
ρ = density
Z = atomic number
E = energy
When is the photoelectric effect most important?
At low energies.
Define Compton scattering
When a photon is scattered by a weakly bound electron the electron is ejected and photon energy is reduced by an amount (depending on the scattering angle).
What is the relation that described the contribution of Compton scattering to the linear attenuation coefficient?
µ = linear attenuation coefficient
ρ = density
